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1Introduction

In the 45-year period of 19301975, the global human population hasincreased by approximately 2 billion, rising to 4 billion. A further pop-ulation increase of 2 billion occurred in the 25-year interval 19752000and population is expected to reach 8 billion by 2020. Population growthand the resultant development of large high-density urban populations,together with parallel global industrialization, have placed major pres-sures on our environment, potentially threatening environmental sus-tainability. This has resulted in the buildup of chemical and biologicalcontaminants throughout the biosphere, but most notably in soils and

sediments.Cases of uncontrolled contamination of soil and other media with

these toxic chemicals emerged, drawing attention to the threats causedby these chemicals to our environment and human health. For example,the Love Canal area in Niagara Falls, New York State was the site of anabandoned canal that was used by the Hooker Chemical Company asa location for disposal of 22,000 tons of PCBs, dioxins, pesticides andother chemical wastes during the 1940s and early 1950s. The site wasthen covered up and acquired by the City of Niagara Falls and subse-quently used as a location for a school and housing. By 1978 toxic chem-icals had leaked from the soil into the basements of area homes and highrates of miscarriages and birth abnormalities were reported amongresidents.

There are many other significant historical examples, including thedioxin crisis in Belgium, Italy and Bhopal. In addition, it was observed

Soil Bioremediation and Phytoremediation An Overview

Owen P. Ward

1

and Ajay Singh

2

1

1 Department of Biology, University of Waterloo, Waterloo, Ontario N2L 3G1,

Canada, e-mail: opward@sciborg.uwaterloo.ca, Tel:+

1-519-8851211 Ext. 2427, Fax:+1-519-74606142 Petrozyme Technologies Inc., 7496 Wellington Road 34, R.R. #3, Guelph, Ontario N1H6H9, Canada

Soil Biology, Volume 1Applied Bioremediation and Phytoremediation(ed by A Singh and O P Ward)

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that many of the toxic chemicals have a tendency to eventually transferfrom solid media to aqueous media and then to bioaccumulate in highlipid-containing species such as fish (Bernard et al. 1999). A furtherconcern is the growing accumulation of pharmaceuticals in the envi-

ronment, mostly excreted in urine (Kozak et al. 2001).The historical foundation for modern environmental biotechnology

lies in the composting of organic wastes into soil fertilizers and condi-tioners. In its broadest sense bioremediation includes biological treat-ment of wastewater, sewage, food and agricultural wastes,contaminatedsoils and groundwater. While its definition is the subject of debate,Prince (1998) defines bioremediation as the process of judiciouslyexploiting biological processes to minimize an unwanted environmen-tal impact; usually it is the removal of a contaminant from the bios-phere. The term intrinsic bioremediation essentially involves takingno action but rather monitoring a natural process of contaminantreduction without intervention. Hence, intrinsic bioremediation canhardly be termed as a technology, but it has met with some success asa low cost approach.

Perhaps more than any other event, the Exxon Valdez oil spill, off thecoast of Alaska, demonstrated the potential for a large-scale applicationof biological processes for cleanup of hydrocarbon-contaminated soil

(bioremediation; Prince et al. 1998). Since that event, the applicationof bioremediation as a biotechnological method for soil remediationhas gained prominence as an alternative or used in combination withphysical or chemical treatment methods.

Soil bioremediation processes may be implemented using a varietyof different engineered configurations ranging from in situ subsurface(unexcavated) processes to application of completely mixed soil slurryreactor systems for treatment of excavated soils. The technology isinterdisciplinary, involving microbiology, engineering, geology, ecology,

chemistry and perhaps other disciplines. The choice of configurationapplied can shift the relative emphasis among the disciplines. Thecommon objective in the various processes is to create the necessaryenvironment to facilitate growth and contaminant degradation by theappropriate biological organisms. Bioremediation has now been usedsuccessfully to remediate many hydrocarbon-contaminated sites as wellas sites containing selected other contaminants (Singh et al. 1999; VanHamme et al. 2003).

It has been estimated that non-biological processes would cost

US$ 750 billion over the next 30 years to remediate all of the known haz-ardous waste sites in the United States (Pimentel et al. 1997). However,the cost would be reduced to $75 billion with the use of bioremediationover the same period of time. According to another estimate, worldwide

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use of bioremediation would cost $14 billion per year compared to theuse of current technologies at $135 billion per year (Hunter-Cevera1998). Various biological and non-biological methods used in soil reme-diation are shown in Figure 1.

While there are many aspects of soil-related biodegradation andbioremediation, this volume will primarily focus on remediation ofchemical contaminants in soil and will not address the issues ofbiowaste recycling.

Soil Bioremediation and Phytoremediation An Overview 3

SOIL REMEDIATIONMETHODS

Coker

Incineration

Cement kiln

Air stripping

Thermal desorption

PhytoremediationBioremediation

Physicalremediation

Solvent extraction

Oxidation-reduction

Precipitation

Neutralization

Encapsulation

Chemicalremediation

Phytoextraction

Phytostimulation

Phytostabilization

Phytotransformation Phytovolatilization Rhizofiltration

Bioreactor

Biopiling

Bioventing

Bioaugmentation Landfarming Natural attenuation

Fig. 1. Soil remediation methods

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2Major Environmental Contaminants

There are two major groups of environmental contaminants, namelychemical and biological wastes that can accumulate in or be trans-mitted via soil as a result of the population expansion and industrialintensification described above. The chemical contaminants can beclassified broadly into two groups, organic and inorganic contaminants.

2.1Chemical Contaminants

The huge expansion of the chemical and petroleum industries in the20th century has resulted in the production of a vast array of chemicalcompounds and materials that have transformed our lives. For example,annual volumes of individual bulk chemicals produced in the UnitedStates range from 520 million metric tons for ethylene,propylene, vinylchloride, benzene and ethylbenzene and from 15 million tons for alarge number of other organic chemicals. Approximately 140 milliontons per annum of synthetic polymers/plastics are produced globally

(Shimao 2001). The global yearly volume of crude oil production isapproximately 72 million barrels per day (West 1996), while the totalworld refining capacity is 74.4 million barrels per day (West 1996). Thelatter huge numbers (about 25 billion barrels per year) indicate the scaleand volumes of refined fuels and other oil-based products produced andused annually. Thus if one assumes that only 1% of these volumes entersthe environment through spills, waste disposal or volatilization, thisamounts to 266 million barrels per annum.

Industrial activities have also resulted in undesired contamination of

soil and other media with heavy metals, so often toxic to human andanimal health. Contamination of soil and solid wastes with high activ-ity radionuclides, such as 235U, 99Tc and 241Pu represents an additionalenvironmental hazard, with the potential for these metals to be radio-toxic to all life forms (Lloyd and Macaskie 2000). Mention should alsobe made here of excessive levels of inorganic fertilizer-related chemi-cals introduced into soil, such as ammonia, nitrates, phosphates, andphosphonates, which accumulate, or lead to the contamination of ourwater courses through run-off or of our air through volatilization.

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2.2Biological Wastes and Contaminants

A second category of waste which threatens the environment is bio-logical wastes, including raw and digested sewage (biosolids), raw anddigested animal manures, and vegetable wastes. While these biologicalwastes have traditionally been recycled into soil for agricultural benefitincreasing urbanization and the continued expansion of cities requiresthat these materials be transported over larger and larger distances forapplication to farm land. Likewise the continuing shift to intensive live-stock farming (factory farms), typically located close to areas of highpopulation, means that traditional land disposal practices of animal

wastes are often rendered uneconomic because of high transport costs.Waste organic products of vegetable and food processing are also can-didates for recycling back into soil, but again high volume intensivefactory operations require large areas of land for recycling. Alternativewaste reduction and beneficial use outlets or other economical disposalapproaches are being sought for these wastes.

An additional problem associated with biological wastes relates to therisks of transmission of infectious diseases when infected materials areapplied to soil. This dimension has become more prominent through

the recent emergence of high profile infectious diseases, such as theprion, BSE; viruses, such as foot and mouth or West Nile and intestinalbacterial pathogens. Infected soils can facilitate disease spread throughdirect or indirect contact with watercourses, plants, animals or humans.

The introduction to or disposal of recombinant organisms in soil isa related concern, given the potential for recombinant strain prolifera-tion, modification or recombinant gene transfer processes in the soil.One small outcome arising from this concern is that the potential to userecombinant organisms in bioremediation processes has been con-

strained. There is surely a need to understand better the nature of soilsas host media for retention, survival and propagation of these biologi-cal organisms/vectors.

3Microbial Transformation of Chemical Contaminants

Processes for metabolism of organic contaminants may be differenti-

ated depending on the organisms ability or otherwise to derive energyor carbon from the transformation for growth. With primary substratesthe cell gains energy and metabolites which can be used for cell main-tenance, division and growth. In the case of secondary substrates the

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